H2S (hydrogen sulfide) in the presence of water can result in damage to carbon steel pipelines in the form of corrosion, cracking, or blistering. The effects of H2S on steel can be classified as those that require external stress, such as with sulphide stress cracking (SSC), and those that do not, such as hydrogen induced cracking (HIC), and corrosion. The presence of carbon dioxide in the sour environment tends to increase the corrosion rate in the steel. It may also increase the susceptibility of the steel to both SSC and HIC, with the effect on HIC being more pronounced.
SSC is characterized by an initial single, straight, trans-granular crack that develops under the application of an external stress, either applied or residual. It may initiate at a corrosion pit (or any other feature that may act as a stress raiser) or through a hydrogen cracking mechanism (e.g., blistering) and then propagate into a brittle fracture perpendicular to the direction of the applied stress. SSC in high-strength steels tends to have considerable branching, while that in low-strength steels shows little. SSC may occur in welds or in the heat-affected zones next to the welds. It usually occurs in carbon steels with tensile strengths greater than 550 megapascals (MPa). However, SSC failures have also occurred in steels with tensile strengths less than 550 MPa. This is primarily due to rapid cooling of weldment, creating localized areas of high hardness.
HIC does not normally occur in welds, but rather in pipe body areas having slag inclusions that act as sites for the collection of diffusing hydrogen. HIC involves two basic forms, straight cracks and stepwise cracks. HIC can occur in low-strength steels with hardness values well below RC 22, with or without the presence of an external stress. Corrosion at the steel surface produces atomic hydrogen, which penetrates and diffuses through the steel until it is trapped at naturally occurring irregularities in the metal, such as non metallic inclusions and laminations. This atomic hydrogen combines into molecular hydrogen and builds up a gas pressure high enough to surpass the yield strength of the steel, forming blisters. Adjoining blisters may propagate as cracks, normally parallel to the rolling direction of the steel.
Another phenomenon associated with HIC is hydrogen embrittlement in steel pipes and welded joints, which generally denotes a loss in ductility of the metal as a result of hydrogen penetration. The susceptibility to hydrogen embrittlement depends on the chemistry and microstructure of the material. Therefore, different regions of the pipe and weldment may embrittle differently due to the presence of hydrogen. Thus, it is desirable to avoid the use of welds in sour applications. In addition, welding may often be difficult in certain environments, whether due to location, harsh conditions, the presence of gases, or otherwise.